Magnetic Random Access Memory (MRAM) is a non-volatile memory technology based on magnetoresistance. One type of MRAM is spin torque transfer MRAM (STT-MRAM), in which a magnetic cell core includes a magnetic tunnel junction (“MTJ”) sub-structure with at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region (e.g., a tunnel barrier material) between. The free region and the fixed region may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) relative to the thickness of the regions. The fixed region includes a magnetic material that has a substantially fixed (e.g., a non-switchable) magnetic orientation. The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration, the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively). In the parallel configuration, the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements (e.g., the fixed region and free region), defining a “0” logic state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements, defining a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core, including the fixed and free regions. The fixed region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current exerts the torque on the free region. When the torque of the spin-polarized electron current is greater than a critical switching current density (Jc) of the free region, the direction of the magnetic orientation of the free region is switched. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enable the write and read operations of the MRAM cell. After switching the magnetic orientation of the free region to achieve the one of the parallel configuration and the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
Switching of the magnetic orientation of the free region of a magnetic memory cell including a MTJ may be affected by the tunnel magnetoresistance (“TMR”) and the resistance area product (“RA”) of the cell. The TMR of a MTJ is a function of the resistance between a top electrode and a bottom electrode, between which the MTJ is disposed, in the high electrical resistance state and the low electrical resistance state. Specifically, the TMR measures the difference between a cell's electrical resistance in the anti-parallel configuration (Rap) and its electrical resistance in the parallel configuration (Rp) to Rp (i.e., TMR=(Rap−Rp)/Rp). Thus, the TMR is equivalent to the change in resistance observed by changing the magnetic state of the free layer. Generally, a MTJ with a homogeneous crystal structure (e.g., a bcc (001) crystal structure), having few structural defects in the microstructure of its magnetic material, has a higher TMR than a MTJ with structural defects. A cell with high TMR may have a high read-out signal, which may speed the reading of the MRAM cell during operation. A higher TMR is preferred for reliable read operation as it will generate a larger signal difference between the on and off states of the cell. In other words, the higher the TMR, the more sensitive the device, and the easier to distinguish logic states of an associated memory cell.
Another significant characteristic of a magnetic memory cell core includes the RA. The RA of a magnetic memory cell is an indication of the voltage used to switch the magnetic orientation of the free region during programming (e.g., the threshold switching voltage). An increase in the RA of a magnetic memory cell may degrade the performance of the cell by utilizing a higher threshold switching voltage, reducing the usable life of the cell. The RA may be decreased by decreasing a thickness of the tunnel barrier material. However, decreasing the thickness of the tunnel barrier material may also decrease the TMR. Thus, although a high TMR and a low RA are desired, in general, an increase in the TMR of a MTJ is obtained at the expense of a higher RA. A conventional MTJ exhibits a TMR of less than about 120% at an RA of greater than about 4 ohm μm2.
Efforts to increase the TMR of a MTJ while maintaining a low RA include attempts to reduce structural defects in the crystal structure of the MTJ. For example, a magnesium oxide tunnel barrier material may be formed at elevated temperatures to produce the tunnel barrier material having stoichiometric proportions and minimal oxygen vacancies or interstitial oxygen. However, the elevated temperatures may undesirably cause an underlying magnetic material to crystallize in an undesired crystal orientation. A mismatch in crystal orientation of the magnetic material and the tunnel barrier material undesirably increases the RA and decreases the TMR of the MTJ. The increase in the RA increases the voltage required to switch the magnetic orientation of the free region during programming, increases the junction resistance, and increases the threshold switching voltage of the device. A decrease in the TMR reduces the effective spin-polarization of the electrons as they pass through the MTJ, reducing tunneling through the MTJ.
Alternatively, the tunnel barrier material may be formed at lower temperatures. However, when the tunnel barrier material is formed at lower temperatures, defects, such as oxygen vacancies and interstitial oxygen atoms, within the tunnel barrier material increase. The atomic defects in the tunnel barrier material may degrade device performance by causing electrons to scatter as they travel through the MTJ and reducing the TMR of the MTJ.